The need for very high purity process gases has always been a serious concern of the semiconductor industry and with the evolution of semiconductor device manufacturing from VSLI (Very Large Scale Integration) to ULSI (Ultra Large Scale Integration) the availability of process gas with increased purity e.g. from parts-per-million (ppm) to parts-per-billion (ppb) is imperative and parts-per-trillion purity requirements are expected for the 1990's.
The manufacture of process gases (e.g. oxygen, argon, hydrogen, nitrogen) of ultra high purity is an established commercial practice as is the delivery of such gases to the location of a semiconductor manufacturing facility. However, at the point-of-delivery, pressurized ultra high purity gas enters a distribution system which connects with semiconductor manufacture process equipment and the distribution system is known to be a potential source of gas contamination and in modern state-of-the-art systems precautions are routinely taken such as the use of electropolished stainless steel tubing and fixtures to provide the smoothest possible surfaces to avoid entrapment of impurities and efforts have been made to effect elimination of leaks and the avoidance of "dead spaces" in the system, i.e. places where contaminants can accumulate and are undiluted and provide a source of re-entrainment or re-entrance of impurities, i.e., a condition known in the art as a "virtual leak".* FNT * A "virtual leak" is so designated since the effect of re-entrainment and re-entrance of accumulated impurities from a "dead space" has the same effect as a leak of impurities into the system.
While the aforementioned problems are recognized and steps taken to avoid unsatisfactory conditions, state-of-the-art systems have not fully addressed these issues, particularly, the elimination of "dead spaces". Also, the important aspects of continuous downstream monitoring for impurities, improved purgability and minimization of welds (to lessen entrapment of impurities) in the distribution system have not been successfully addressed. In the recent publication "Design and Performance of the Bulk Gas Distribution System in The Advanced Semiconductor Technology Center (ASTC)", Bradley Todd--Proceedings of Microcontamination Conference, October, 1989, the problems of controlling contamination in distribution systems was presented and a system described in which problems were addressed; however, the problem of contaminant accumulation in "dead space" in the laterals branching from the distribution system main line was not addressed. Similarly, the publication "Ultra Clean Gas Delivery System", Kenneth R. Grosser--Technical Proceedings of Semcon/East, September, 1989 recognizes the problems associated with dead zones and discloses a system in which a loop was used in the main lines to maintain flow in major elements of the disclosed system but the matter of "dead space" in laterals branching from the main line was not addressed. Also, in the publication "Examining Performance of Ultra-High- Purity Gas, Water, and Chemical Delivery Subsystems," Tadahiro Ohmi, Yasuhiko Kasama, Kazuhiko Sugiyama, Yasumitsu Mizuguchi, Tasuyuki Yagi, Hitoshi Inaba, and Michiya Kawakami Microcontamination, March 1990 the problem of gas stagnation is fully recognized and a system with constant flow in gas lines is described, and also described is the use of an integrated valve to supply gas to four pieces of process equipment which is provided with a constant purge line so that the lines between the integrated valve and the process equipment inlets can be purged with small amounts of gas. It is fully accepted in the art that prevention of contamination in a gas distribution system is a critical concern of semiconductor manufacturers and the problem of contamination resulting from "dead space" in the distribution system is fully recognized but as yet no comprehensive solution has been presented.